Physiology of hearing Flashcards
Functions of hearing
- Alerting to dangers
- Localising objects
- Recognition
- Communication via speech
Frequency range of human hearing
- Approx 20-20,000 Hz
What hearing intensity can lead to permanent hearing damage
> 90 dB
Bones of the ear
- Malleus
- Incus
- Stapes
What does the impedance matching device do
- Increases pressure 45x, by the ratio of tympanic membrane and oval window areas, and to a lesser extent by the lever action of the middle ear ossicles
Purpose of the impedance matching device
- Prevents sound from being reflected back from the fluid-filled cochlea
What is otitis media(and glue ear)
• Infection or inflammation of middle ear
○ Usually self-limiting
• Secretory form with effusion
○ “Glue ear”
If chronic causes a conductive hearing loss
○ May need draining
- Grommets
Who are commonly affected by otitis media
- Common in children
- Often from upper respiratory tract infection
What is otosclerosis
- Fusion of stapes with oval window
- Can be fixed by surgery
What is the cochlea
- Is a long, coiled, fluid filled tube
How are the ends of the cochlea tuned
- Basal end is tuned to high frequency sound
- Apical end is tuned to low frequency sounds
Tubes in the cross-section of the cochlear duct
- Scala vestibuli
- Scala media
- Scala tympani
What is the scala vestibuli connected to
- Oval window
Feature of scala media
- Is a separate chamber
What is the scala tympani connected to
- Connected to round window
How do the SV and ST communicate
- Via the helicotrema at apex of cochlea
Why do scala vestibuli and scala tympani contain
- Contain perilymph
Content of perilymph
- Extracellular fluid with high Na+ and low K+
What does the scala media contain
- Contains endolymph
Content of endolymph
- Rich in K+ and low in Na+
What is the endolymph produced by
- Stria vascularis
Electrical potential of endolymph
+80mV
What is organ of corti
- Is the receptor organ for hearing and is located in the mammalian cochlea
- This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses’ action potential
Location of the organ of corti
- Organ of Corti is located in the scala media of the cochlea of the inner ear between the vestibular duct and the tympanic duct
Purpose of the organ of corti
The function of the organ of Corti is to transduce auditory signals and minimise the hair cells’ extraction of sound energy
- Detects the sound induced motions of the basilar membrane
What are the types sensory hair cells in the organ of corti
- Inner hair cells
- Outer hair cells
What are the apical and basolateral membrane of hair cells bathed in
- Apical membrane of hair cells is bathed in endolymph
- Basolateral membrane of hair cells is bathed in perilymph
What are inner hair cells innervated by
- Afferent nerve fibres
What are outer hair cells innervated by
- Mainly innervated by efferent nerve fibres
How many hair cells are there in each human cochlea
- Only 15,000
- Not regenerated after loss
How does mechanotransduction in hair cells occur
Deflection of the hair bundle opens non-selective cation channels, the mechano-electrical transducer (MET) channels, at the lower end of the tip links, between neigbouring stereocilia (‘hairs’)
K+, the major cation in endolymph enters and depolarises the hair cell, driven by its electro-(chemical) gradient [the electrical gradients is +120 to +140 mV; there is little or no chemical gradient]; Ca2+ also enters and causes adaptation
- Voltage gated Ca2+ channels open, Ca2+ triggers vesicle release
Effect of Ca2+ release on hair cells
- Afferent nerve fibres are activated
- Inner hair cells are sensory
- Outer hair cells are sensori-motor cells
Electromotility of outer hair cells
- Outer hair cells amplify basilar membrane motion
- Depolarise - shorten; hyperpolarise - lengthen
Effect of prestin
- A modified anion exchanger in the basolateral membrane, is the OHC motor
Afferent innervation of the cochlea
Neurons in cochlear (spiral) ganglion innervate hair cells and project axons to the brain via the auditory branch of the VIIIth nerve
What is each inner hair cell innervated by
- Each inner hair cell is innervated by axons from 10-20 Type I spiral neurons that signal the reception of sound over a wide range of intensities to the brain
What are the outer hair cells innervated by
- Outer hair cells are innervated by type II spiral neurons that signal the reception of painfully loud sound that causes cochlear damage to the brain
Efferent innervation of the cochlea
- Efferent fibres from the medial olive innervate the outer hair cells directly
- Efferent fibres from the lateral olive synapse on the type I afferent fibres
What does activation of efferent system cause
- Modification of the sensitivity of the cochlea
How does noise contribute to sensorineural hearing loss
- Physical effects on hair bundle structure mitochondrial damage, cytotoxic free radicals glutamate excitotoxicity
How does ageing cause sensorineural hearing loss
- Ageing(presbyacusis)
- 30% of population over age of 70 have significant hearing loss hair cells, stria vascularis, cochlear ganglion
ototoxic drugs that cause sensorineural hearing loss
- Aminoglycoside antibiotics, cisplatin, loop diurectics, salicylate, solvents
Sensorineural hearing loss - genetic impact
Genetic mutations
• High frequency, 1:2000 of live births
• Syndromic and non-syndromic
• >50 deafness genes identified, 80 additional loci
• ~50% of congenital deafness caused by mutations in
gap junction genes
Number of deafness genes identified so far
> 50 deafness genes identified
- 80 additional loci
What percentage of congenital deafness is caused by mutations
- 50% of congenital deafness is caused by mutations in gap junction genes
Targets of deafness genes in the cochlea
- Tight junctions
- Gap junctions
- Afferent synapse
- Tectorial membrane
- Stria vascularis
- Transduction complex
What are cochlear implants
- Surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf
Results of cochlear implants
Results often good enough to recognise and comprehend speech
- Maximum 24 channels to substitute for 15,000 hair cells
- Speech is reported to sound ‘robotic’
- Music sounds awful
Central auditory system
- Cochlear nucleus –> superior olivary complex –> nuclei of the lateral lemniscus –> inferior colliculus –> medial geniculate body –> primary auditory cortex
Where does parallel processing start
- Cochlear nucleus
Type of innervation provided by auditory nerve fibres from cochlear ganglion
- Auditory nerve fibres from cochlear ganglion innervate many types of neuron
- Neurons extract info about level, onset and timing of sounds
What are the two binaural cues used to localise sounds in space in the superior olivary complex
- Interaural level differences are detected in the lateral superior olive(LSO)
- Interaural time differences are detected in the medial superior olive(MSO)
What is the inferior colliculus
- Obligatory synaptic station for all afferents
- Laminar organisation in ICC, iso-frequency sheets
Purpose of the inferior colliculus
- Combines complex frequency and amplitude analysis of DCN(dorsal cochlear nucleus)
- With info on sound localization from SOC(Superior olivary complex)
- May encode complexity and localization of sounds
- Auditory reflex centre; reflexive orientation to stimuli
Location of the primary auditory cortex
- Primary auditory cortex is located on upper surface of temporal lobe
What do lesions in auditory cortex cause
- Lesions in auditory cortex cause defects in: sound localisation, discrimination of temporal pattern, intelligibility of speech
What do lesions in broca’s and wernicke’s areas cause
- Impair the production and comprehension of speech
